Bath vent ducting

[mkgrady]

Not technically an electrical question but many electricians do this as part of a bath fan job.

I'm going to replace three fan/light units in a customers house. The fans are on the top floor with unfinished attic above. I found the existing 4 inch duct from each fan ran into one 4 inch duct with a couple of "Y" fittings. I told him each should have it's own duct and offered to install them. Not a difficult job.

My question is, when he asks me why it can't stay the way it is I don't know enough about the subject to provide a good answer.

As a side note. Incredibly, the one duct that runs to the outside was sealed off with a piece of sheet metal where there should have been a flap termination thingamajig. Because of this, each bathroom has been blowing air from one bath to the others. All the ducts are full of dust/debris, so I plan to replace them all.

 

[eprice]

My question is, when he asks me why it can't stay the way it is I don't know enough about the subject to provide a good answer.

I think the answer is manufacturers installation instructions. I doubt that any manufacturers instructions would allow venting as you describe the existing installation to be vented.

 

[Dennis Alwon]

If the ducts are sized correctly it should work however two 4" ducts going into one 4" duct is probably not good enough. You would probably need dampers at the "Y" also.

 

[mkgrady]

If the ducts are sized correctly it should work however two 4" ducts going into one 4" duct is probably not good enough. You would probably need dampers at the "Y" also.

It's actually three into one duct. All are four inch, including the main one to the outside.

 

[electricmanscott]

All three fans running at the same time into one duct will kill the air movement.

 

[BILLY101]

Residential bathroom fans require a dedicated unobstructed discharge to obtain the rated CFM. A wall hood with a back-draft damper is required to keep varments and drafts out.

BILLY

 

[stevebea]

Not technically an electrical question but many electricians do this as part of a bath fan job.

I'm going to replace three fan/light units in a customers house. The fans are on the top floor with unfinished attic above. I found the existing 4 inch duct from each fan ran into one 4 inch duct with a couple of "Y" fittings. I told him each should have it's own duct and offered to install them. Not a difficult job.

My question is, when he asks me why it can't stay the way it is I don't know enough about the subject to provide a good answer.

As a side note. Incredibly, the one duct that runs to the outside was sealed off with a piece of sheet metal where there should have been a flap termination thingamajig. Because of this, each bathroom has been blowing air from one bath to the others. All the ducts are full of dust/debris, so I plan to replace them all.

Nutone recommends a dedicated duct run for each fan, however if you choose to use a "Y" fitting the cross section of the common duct must equal or be greater than the cross section of the two individual ducts. Example, two 4" ducts have a cross section of 25" so you have to go to a 6" duct which has a cross section of 28".